1. Field of the Invention
The field of the invention is that of solid-state ring lasers, also called laser gyros. Almost all laser gyros use a gaseous amplifying medium, which is usually a mixture of helium and neon. However, it is possible to use a laser gyro having a solid-state amplifying medium in which the gaseous amplifying medium is replaced with a solid-state element, for example a neodymium-doped YAG (yttrium aluminium garnet) matrix.
2. Description of the Prior Art
The operating principle of a laser gyro is based on the Sagnac effect of a bidirectional ring laser cavity undergoing a rotational movement. The Sagnac effect induces a frequency difference Q between two counterpropagating optical emission modes that propagate in opposite directions inside the cavity. In the solid-state media normally used, including Nd:YAG, the modes propagating in opposite directions share the same amplifying atoms. The gain is therefore said to be homogeneous. When the two counterpropagating modes have the same or very similar frequencies, the interference signal that results therefrom is a standing wave, which may possibly be moving. The atoms of the gain medium participate more in the stimulated emission process when they are close to an antinode of the standing wave and less when they are close to a node. This therefore creates, in the gain medium, a population inversion grating written by the standing wave. This grating continues to exist as long as the frequencies of the two counterpropagating modes are sufficiently close. Its contrast is lower the greater the frequency difference compared with the inverse of the lifetime of the excited level. It has been shown that this population inversion grating has deleterious effects for gyro measurements, for two main reasons:                it exacerbates the competition between the counterpropagating modes, preventing in most cases the beat regime, which is the operating regime to be established in a gyrometer; and        it induces a non-linearity in the frequency response when the laser is rotating, thereby degrading the inertial performance.        
The first of these points may be dealt with by various techniques based for example on electronic feedback devices. An optical device placed in the cavity acts differently on the intensity of the modes according to their direction of propagation. These devices are generally based on non-reciprocal optical effects, such as the Faraday effect.
However, the devices used to deal with the problem of intermodal competition perform less well at low rotation speeds and do not in general get round the problem of non-linearity of the frequency response of the laser gyro. This problem may for example be solved by introducing a strong frequency bias between the two counterpropagating modes. It is then necessary to control the stability of the bias used, otherwise the inertial performance is limited. It is also possible to eliminate the standing wave in the gain medium and the population inversion grating generated by this wave by ensuring that the polarization states are orthogonal when they interact with the crystal. The latter technique requires the birefringence in the cavity to be controlled, which means it is difficult to use when high inertial performance is required.